1. Field of the Invention
This invention relates to composite panels. More specifically, the invention is a composite panel that has subsonic transverse wave speed characteristics in regions thereof for reduced sound radiation efficiency and increased sound power transmission loss.
2. Description of the Related Art
Composite materials are used in many construction applications (e.g., structures, aircraft, trains, vehicles, industrial machines, etc.) because of their light weight and strength. The materials are frequently formed into what are known as composite panels where two sheets of one type of material are sandwiched about another type of core material. For example, one type of composite panel has two sheets of a material such as graphite-epoxy, epoxy, fiberglass or aluminum sandwiched about a honeycomb core made from materials such as NOMEX, aluminum or paper. The resulting composite panel is light and stiffer than any of its component parts. However, as can be the case with most lightweight and stiff materials, sound can be radiated very efficiently because the transverse wave speed through the panel can be greater than the speed of sound in air. In other words, the composite panel has a supersonic transverse wave speed. If the composite panel is to be used to define a human-occupied interior space, noise radiated by the composite panel into the interior space may be unacceptable. Current methods of addressing this noise problem have involved the addition of noise control material to the composite panel such that the noise-controlled composite panel is characterized by a subsonic transverse wave speed. Suggested additions include a limp mass (e.g., lead vinyl) applied to one or both of the composite panel's face sheets and/or the inclusion of foam within the composite panel's core in the case of a honeycomb core. However, the extra noise-control material adds cost and weight to the composite panel.